Radio Frequency Identification (RFID) systems typically include RFID tags and RFID readers. RFID readers are also known as RFID reader/writers or RFID interrogators. RFID systems can be used in many ways for locating and identifying objects to which the tags are attached. RFID systems are particularly useful in product-related and service-related industries for tracking objects being processed, inventoried, or handled. In such cases, an RFID tag is usually attached to an individual item, or to its package.
In principle, RFID techniques entail using an RFID reader to interrogate one or more RFID tags. The reader transmitting a Radio Frequency (RF) wave performs the interrogation. The RF wave is typically electromagnetic, at least in the far field. The RF wave can also be predominantly electric or magnetic in the near field.
A tag that senses the interrogating RF wave responds by transmitting back another RF wave. The tag generates the transmitted back RF wave either originally, or by reflecting back a portion of the interrogating RF wave in a process known as backscatter. Backscatter may take place in a number of ways.
The reflected-back RF wave may further encode data stored internally in the tag, such as a number. The response is demodulated and decoded by the reader, which thereby identifies, counts, or otherwise interacts with the associated item. The decoded data can denote a serial number, a price, a date, a destination, other attribute(s), any combination of attributes, and so on.
An RFID tag typically includes an antenna and an RFID integrated circuit (IC) including a radio section, a power management section, and frequently a logical section, a memory, or both. In some RFID tags the power management section employs an energy storage device, such as a battery. RFID tags with an energy storage device are known as active or battery-assisted tags. Advances in semiconductor technology have miniaturized the electronics so much that an RFID tag can be powered solely by the RF signal it receives. Such RFID tags do not include an energy storage device such as a battery, and are called passive tags. Regardless of the type, all tags typically store or buffer some energy temporarily in passive storage devices such as capacitors.
In a typical RFID manufacturing process, RFID ICs are manufactured on semiconductor wafers. The RFID ICs are diced from the wafer and then attached to antennas, typically formed on a substrate. The substrate and antenna (together an inlay), and IC together comprise the RFID tag. The IC is electrically coupled to the antenna by pressing the IC onto the antenna such that bumps formed on antenna connections of the RFID IC establish a connection to the antenna. Pick and place machines pick an IC, position it on the antenna, and press the two together.
Optionally, RFID ICs may first be attached to a strap with a strap substrate and one or more strap pads, where the antenna pads of the ICs are electrically coupled to the strap pad(s). The strap itself may then be placed onto an inlay with an antenna such that the strap pad(s) (and therefore the antenna pads of the IC) are electrically coupled to the antenna. In these embodiments, the IC, strap, and inlay together comprise the RFID tag.
In some instances the machines may not apply consistent force during the attachment process. Variations in the force may result in differing mounting distances between the RFID IC and the antenna, which in turn causes a mounting capacitance between the RFID IC and the antenna to vary from tag to tag. The mounting capacitance affects the performance of the RFID tag. Tags with different mounting capacitance have undesired performance variations in RFID applications.